In optical transmission systems, cables, such as fiber-optic cables, are used to transmit information. In some systems, the cables extend from an optical line terminal (“OLT”) or other optical device to one or more optical network units (“ONUs”). Optical signals of a certain group of wavelengths are transmitted (upstream signal) from the optical device to the ONUs. Additionally, optical signals of a certain group of wavelengths, which may be different from the group of wavelengths of the transmitted optical signals, are received (downstream signal) from the ONUs at the optical device.
In order for the optical device to transmit or receive at a desired wavelength from among the group of wavelengths, the device must be tuned. For an upstream signal, the signal generator (e.g., laser) of the device may be tunable to provide what is effectively a “pre-filtered” signal. Alternatively, the laser may transmit an optical signal to a destination ONU with an optical transmission made up of multiple wavelengths (with minimal or no filtering applied), and the optical signal can be filtered from the transmission at the destination ONU. Similarly, the ONU may transmit a multiple-wavelength optical signal to the optical device, and the signal can be filtered at the optical device. In order to filter such a signal, a tunable optical filter may be provided. To accommodate for transmitting and receiving both “pre-filtered” and non-“pre-filtered” optical signals, the optical device may include either or both of a laser and optical filter that can be tuned to a frequency that corresponds to the desired wavelength. Conventionally, this tuning is performed by changing the temperature of the laser/optical filter. Thus, tuning at the optical device may be performed using a temperature dependent tunable element.
As passive optical networks (PONs) become increasingly faster, it becomes increasingly more important for the tunable element to be tuned at a high speed. At the same time, while it is important to be able to tune the tunable element at a high speed, it is also necessary for the tuning to be precise and accurate in order to minimize attenuation of the desired downstream or upstream signal (e.g., as it passes through the cable or filter). In other words, it is desirable to both rapidly and precisely change a temperature of the tunable element. It is further desirable for the optical device to have a relatively compact size, and to minimize the cost of the temperature control components, as well as the cost of their installation.
It is generally known that a thermoelectric cooler (TEC) may be used to stabilize the temperature of an optical device in order to stabilize the wavelength of the upstream or filtered downstream optical signal. However, the TEC changes the temperature of the device at a relatively slow rate. The TEC would need to be able to change the temperature of the tunable element at a faster rate in order to be a suitable tuning mechanism.
In the case of a laser that is operated in burst-mode, it is also generally known that a heater may be used to stabilize the temperature of the laser, thereby stabilizing the wavelength of the light emitted from the laser. Without temperature stabilization achieved by the heater, the wavelength of light emitted from the laser could drift due to temperature fluctuations in the laser caused by the abrupt transitions between on (emitting light) and off (not emitting light) modes. Generally, the heater is used to keep the laser warm while it is off so that there is no temperature fluctuation when the laser is turned on. However, operating the heater in an efficient manner is challenging. It is only necessary for the laser to be kept warm just before it is turned on; therefore, it is not necessary to operate the heater the entire time the laser is off. But because of the unpredictable nature of status transitions when operating the laser in burst-mode, it is not known when the laser will turn on. Additionally, the amount of time for which the laser is off varies from cycle to cycle, making the amount of time and energy needed to warm the laser inconsistent as well.